How to test click commands that expect files

Question

I have a click command that does something with a file:

import click

@click.command()
@click.argument("file", type=click.File("r"))
def foo(file):
    print(file.read())

And I'd like to test it without creating temporary files, but I don't know which path to give the runner and can't find examples on the web. Something like this would be nice:

from click.testing import CliRunner

from magic_lib import magic_file

def test_foo():
    runner = CliRunner()
    fake_file = magic_file("Hello, world!")
    result = runner.invoke(foo, [fake_file.location])
    assert result.output == "Hello, world!\n"

Any way I can make click.File understand where I want it to look?

Aran-Fey · Accepted Answer · 2019-09-25 07:44:13Z

1

You can use pyfakefs. The usage depends on your testing framework; it's easiest to use with pytest because it automatically provides an fs fixture:

def test_foo(fs):
    fake_file = fs.create_file('/foo/bar', contents="Hello, world!")

    runner = CliRunner()
    result = runner.invoke(foo, [fake_file.path])
    assert result.output == "Hello, world!\n"

P.S.: Because the print in foo adds a newline, the file has to be created without \n at the end for the test to work.

edited Sep 25, 2019 at 7:44

answered Sep 24, 2019 at 14:39

Aran-Fey

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4 Comments

Arne Over a year ago

"pyfakefs automatically adds a newline to the file" I think that's from the click function, since it calls print.

Arne Over a year ago

Funny thing, I just found an example solution using click's own test object here, but yours is way leaner and allows me to re-use my invoke-fixture, so I won't bother writing an answer featuring that one.

Joaquin Over a year ago

@Arne I think you should reconsider it. IMO The optimal solution is to use isolated_filesystem

Arne Over a year ago

@FcknGioconda why do you think click's is better? As far as I can see, pyfakefs does the same thing, but once I learned it I can also use it in all my other projects that don't use click.

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